Fuel cells have been proposed for many applications including electrical vehicular power plants to replace internal combustion engines. Hydrogen is often used as the fuel and is supplied to the fuel cell's anode. Oxygen (as air) is the cell's oxidant and is supplied to the cell's cathode.
The hydrogen used in the fuel cell can be derived from the reforming of methanol in a catalytic reactor known as a reformer. In the methanol reforming process, methanol and water (vapors) are ideally reacted under isothermal conditions to generate hydrogen and carbon dioxide according to the following endothermic reaction: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2
This reaction is carried out within a reformer that is heated by exhaust gases from a methanol-fired or hydrogen-fired combuster, and yields a reformate gas comprising hydrogen, carbon dioxide, carbon monoxide, and water. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh, and one such combuster is described in copending United States patent applications U.S. Ser. No. 08/975,422 (abandoned) and Ser. No. 08/980,087 now U.S. Pat. No. 6,077,620, filed in the name of William Pettit in November 1997, and assigned to General Motors Corporation, assignee of the present invention. Carbon monoxide is contained in the H.sub.2 -rich reformate/effluent exiting the reformer, and must be removed, or reduced to very low concentrations (i.e., less than about 20 ppm), which are nontoxic to the catalyst in the anode of the fuel cell.
It is known that the carbon monoxide, CO, content of the reformate can be reduced by the a so-called "water-gas shift" reaction which can take place within the reformer itself (depending on the operating conditions of the reformer), or in a separate shift reactor downstream from the reformer. In the water-gas shift reaction, water (i.e., steam) reacts with the carbon monoxide according to the following ideal endothermic shift reaction: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
Some CO still survives the water-gas shift reaction and needs to be reduced to below about 20 ppm before the reformate can be supplied to the fuel cell. It is known to further reduce the CO content of H.sub.2 -rich reformate by reacting it with air in a so-called "PrOx" (i.e., preferential oxidation) reaction carried out in a catalytic PrOx reactor. In the PrOx reactor, air preferentially oxidizes the CO in the presence of the H.sub.2, but without consuming/oxidizing substantial quantities of the H.sub.2. The PrOx reaction is exothermic and proceeds as follows: EQU CO+1/2O.sub.2.fwdarw.CO.sub.2
The PrOx reactor effluent is then supplied to the fuel cell.
For vehicular power plants, these reactions must be carried out as efficiently as possible and in the most compact space possible. This requires optimal use of available heat to maintain reactor temperatures at their operating temperatures.